Browsers

My recommendations:

Mac OS X: Safari
MS Windows: Safari
Linux: Firefox, Opera

Versions of all "major" rendering engines:

browser latest β latest α platforms
Safari Webkit nightly Mac, Windows
Firefox Shiretoko Minefield nightly Mac, Windows, Linux
Opera Peregrine blog Mac, Windows, Linux
Internet Explorer Explorer β Windows

Mailers

My recommendations:

Mac OS X: Mail
other OS's: Thunderbird, Opera

(Don't even think about using one named after a tree.)